1. Field of the Invention
This invention relates generally to a method for continuously determining cardiac output from a pulse contour curve based on measured arterial pulse data.
2. Description of the Prior Art
The traditional method for continuously determining cardiac output (CO) is the thermodilution method which requires inserting an instrument directly into the body of the patient. There is a safety risk with the thermodilution method and so others have suggested methods for determining CO indirectly from outside the body. Until the present invention, however, none of the non-invasive methods have achieved accuracy close enough to the accuracy of the thermodilution method.
Warner et al, Computer-based monitoring of cardiovascular functions in postoperative patients, Circulation 27 (suppl 2) II 68-74, 1968, suggested computation of cardiac output via computer analysis of the central aortic pulse-wave contour.
Warner U.S. Pat. No. 4,834,107 discloses a non-invasive method for determining heart-related parameters in patients which determines pulse pressure, time constant of the arterial system, systolic and diastolic pressure, peripheral resistance, cardiac output, and mean arterial blood pressure. Warner uses volume data rather than pulse data, and does not recognize the need to adjust for inaccuracies from reflective waves, nor, of course, does Warner describe a method for making the appropriate adjustments. Further, the Warner method apparently must recalibrate for each patient, and does not provide a method for factory calibration while achieving very accurate cardiac output information.
Others have suggested improvements to Warner et al's method; see, for example:
English, et al, Comparison of Aortic Pulse-wave Contour Analysis and Thermodilution Methods of Measuring Cardiac Output during Anesthesia in the Dog, Anesthesiology 52:56-61, 1980;
Wesseling et al, A simple device for the continuous measurement of cardiac output, Adv. Cardiovasc. Phys., 5 (part II):16-52, 1983;
Alicandri et al, Possibility of cardiac output monitoring from the intra-arterial blood pressure profile, Clin. and Exper. Theory and Practice, A7 (2 & 3), 345-353, 1985;
De Meersman, New Noninvasive Computerized Method for the Area Measurement of the Dicrotic Notch, Comput. Biol. Med., 19, No. 3:189-195, 1989;
Wesseling et al, Arterial haemodynamic parameters derived from noninvasively recorded pulsewaves, using parameter estimation, Medical and Biological Engineering, Nov. 1973:724-732; and
Stern et al, Anesthesiology, 73, 3A, September 1990, A455, described a new model (pulse contour) for a peripheral arterial waveform analysis describing the fluid and wall motion of the arterial system. The algorithm is described as processing the area under the arterial waveform based on impedance (Zao) along the arterial tree. Stern et al appear to be using the Wesserling et al method, citing "A Simple Device for the Continuous Measurement of Cardiac Output," Adv. Cardiovasc. Phys., vol. 5 (Part II):16-52. Karger, Basel 1983. Stern et al require frequent recalibrations and yet they still only report achieving a correlation of 0.65 and a standard deviation of .+-.0.8 liters.
In general, the prior methods requires calibration for each patient. In no case is a correction for reflected pulse waves provided.